RF amplifier circuits are utilized for a variety of different purposes including RF communication applications, such as cellular telephone and other wireless communication applications. Generally, all practical RF power amplifiers will add unintentional distortion to the signals that they are intended to amplify. To address such undesired signal distortion, various different linearization processes are known and may be used to minimize the distortion. One such linearization process that is commonly used is termed “feed-forward” linearization. Generally, various feed-forward techniques are known to persons of ordinary skill in the art.
A typical feed-forward amplifier topology incorporates a main amplifier located within a carrier cancellation loop of the circuit, and an error amplifier located within a distortion cancellation loop of that circuit. Briefly, an input signal is split with a portion of the signal going to the main amplifier and another portion going along a non-amplified path. The main amplifier then amplifies the signal of interest, but, as noted above, also introduces unintentional distortion to that amplified signal. The unamplified signal portion is then constructively subtracted from the amplified/distorted signal, to effectively cancel the signal of interest and leave only the distortion signal. Since the signal of interest is often an RF carrier signal, the first loop of a feed-forward amplifier is generally referred to as a carrier cancellation loop. The distortion signal that remains from the carrier cancellation loop is then further processed in the distortion-cancellation loop. To that end, in the distortion-cancellation loop, the distortion signals are amplified and then destructively added to the amplified/distorted signal along the main amplifier path in such a way as to effectively remove the distortion, thereby leaving only the amplified signal of interest. In short, in a feed-forward amplifier, the distortion is isolated from the signal of interest and then added back to the main amplified signal in a subtractive sense to effectively remove the distortion from the main signal. Therefore, the second loop that provides the output of the feed-forward amplifier is often referred to as the distortion cancellation loop.
To optimize a feed forward amplifier and its ability to reduce distortion in the amplified signal, a test signal, such as a pilot tone, is injected at the input to the main amplifier. The pilot tone is generated by a local oscillator (LO) and modulated using a modulation source and a mixer. The injected pilot tone represents unintentional distortion generated by the main amplifier. To optimize the linearization process, the linerarization circuit, and settings therein, are optimized so that the injected pilot tone is minimized or eliminated at the output of the feed-forward amplifier.
Optimization is typically accomplished by sampling the RF output and then demodulating the injected pilot tone to isolate the pilot tone. Usually, the same local oscillator (LO) is used in the demodulation process to guarantee proper alignment of the demodulation carrier and to recover the pilot tone with the same frequency and characteristics as the original modulation source. The demodulated pilot tone is then filtered by a band pass filter to isolate the pilot tone and remove any other additional undesired signals. The filtered pilot tone, which represents the distortion that still exists, is then amplified and detected to produce gain and phase control signals that are proportional to the level of the detected pilot tone. Those gain and phase control signals are then used to control the input to the error amplifier, and thus minimize the pilot tone. In that way, the amplification process between the input and the output of the feed-forward amplifier becomes more linear as the level of the pilot tone that is measured by the detector is minimized.
As such, the ability to discern the pilot tone level in the detector circuit limits the extent of the linearization process. As such, the quality of the linearization utilizing the pilot tone signal as a test signal depends upon the dynamic range of the detected pilot tone that is sampled from the RF output and the signal-to-noise ratio of the detected pilot tone signal to the background noise in the detector. In a typical feed-forward optimization scheme, analog filtering techniques are employed and implement band pass filter elements to eliminate all signals other than the pilot tone signal that is sampled from the RF output. For such optimization it is desirable to use a very narrow band pass filter to eliminate all other extraneous signals other than the pilot tone that is sampled from the output in order to achieve better signal-to-noise ratio and greater dynamic range of the output pilot tone sample. The linearization process functions to continually reduce the amplitude of the pilot tone until it is indiscernible from background noise. That is why narrow band filtering is very important. In such filtering, a compromise occurs between the filter band of the filter device and the filter center frequency value.
More specifically, the Quality factor of a filter (Q-factor) is commonly used to describe the ratio of band width to center frequency for that filter. A narrow band pass filter (high Q) requires very high tolerance component values to keep the narrow band width filter aligned to the pilot tone sample frequency. However, practical limits on component costs and reproducibility restrict the value of Q to around 10. In a typical system, dynamic range considerations require an absolute bandwidth on the order of 100 Hz. If Q is limited to 10, the center frequency will be on the order of 1 kHz. At this frequency, typical LC filters have various drawbacks. For example, they are bulky, expensive, microphonic and are subject to drift. Active filters might be utilized. However, narrow bandwidth active filters are generally quite noisy.
As such, implementing band pass filters with high Q values will result in center frequency drift which affects the pilot tone amplitude. With an improperly centered band pass filter that varies as component values change with temperature, the pilot amplitude variation is interpreted as a change in the linearization metric for the feed-forward amplifier, and, therefore, an undesired operation of the linearization control loop. On the other hand, band pass filters that are implemented with a low Q will remain properly aligned to the output pilot tone sample; however, they will lack dynamic range and have a worse signal-to-noise ratio which limits the quality of the linearization to only modest levels.
Accordingly, it is desirable to improve the operation of feed-forward amplifiers and more specifically, to improve the linearization process for optimizing such feed-forward amplifiers. Still further it is desirable to address the filtering shortcomings associated with the prior art which limit the quality of the linearization process and thus limit the overall optimization and operation of the feed-forward amplifier. These concerns, and others are addressed by the present invention as set forth in more detail below.